[rootnode /]# mdadm --add /dev/md2 /dev/sda3
mdadm: /dev/sda3 reports being an active member for /dev/md2, but a --re-add fails.
mdadm: not performing --add as that would convert /dev/sda3 in to a spare.
mdadm: To make this a spare, use "mdadm --zero-superblock /dev/sda3" first.

and I cant re-add the 2nd device,
If I run mdadm --zero-superblock /dev/sda3
would what erase my data from the partion? or would that create any other damage?

I've searched the manual which says:

--zero-superblock
If the device contains a valid md superblock, the block is overwritten with zeros.
With --force the block where the superblock would be is overwritten even if it doesn't appear to be valid

but i;m not sure if this erases data and since the partition mounts to / in which all the data is, i'm scared to tried it without verification.